An image sensor is a device which senses light of an optical image and then converts the optical image into electrical signals. Image sensors have evolved from video camera tubes to Charge-Coupled Devices (CCDs) and Complementary Metal-Oxide Semiconductor (CMOS) devices. CCD and CMOS image sensors are widely used in various applications such as consumer electronics (e.g., digital single-lens reflex (DSLR) cameras, cameras for mobile telephones, webcams and optical mice) and industrial applications (e.g., machine vision, security surveillance, automotive safety and control).
CCDs have historically been the choice for image sensors in high performance digital cameras because they were able to achieve higher noise performance, with a lower dark current than their CMOS counterparts. However, recent advancements in CMOS noise performance have enabled CMOS image sensors to become prominent in high performance cameras.
CMOS image sensors are made up of an array of pixels. Each pixel, also referred to as a pixel structure, has a photo detector which converts light energy into electrical charge. The electrical charge is then transferred and converted into a voltage signal using the transistors in the pixel structure. The voltage signal is then converted into digital signal by an analog-to-digital converter (ADC) in the read circuit of the image sensor.